Erythropoietin is known to act on erythroblastic progenitor cells in bone marrow to promote their differentiation into red blood cells. However, this substance has been made available only by the process of extracting and purifying it from human urine, thus preventing its clinical application on a large scale. With rapid progresses in recombinant DNA technology in recent years, it became possible to mass-produce erythropoietin analogs which are similar to the natural erythropoietin of human origin. These substances contributed dramatically to the improvement of renal anemic symptoms in patients with those diseases which are suspected to be chiefly associated with compromised erythropoietin production, and a further expansion of the clinical utility of erythropoietin is foreseen.
Today, erythropoietin is clinically administered to patients by the intravenous route but since the excretion of this substance after administration is fairly rapid, it must be administered as often as twice to three times a week. Moreover, the elevation of hemoglobin concentration and of hematocrit level is preferably gradual and any abrupt rises in these parameters would increase adverse reactions such as hypertension. Thus, side effect is a serious concern if a high blood concentration is reached immediately after administration, as is inevitable in the case of intravenous injection. Therefore, some proposals have been made for overcoming this disadvantage accompanying the administration of physiologically active peptides.
Long-acting preparations of a medicine are generally designed to maintain a sustained blood level of the medicine in humoral fluid but according to the mechanism of development of pharmacologic effects involved, medicine of this kind may generally be classified into two categories. The first category (a) is such that the pharmacologic effect of the medicine is not very dependent on effective humoral concentration and an excess of pharmacologic effect is not so detrimental to the recipient's physiology. The second category (b) is such that the pharmacologic effect of the medicine is dependent on effective humoral concentration and because the abrupt onset of its pharmacologic effect or an excessive pharmacologic effect is harmful to the body, the dosage must be adjusted from time to time.
Because of its dramatic pharmacologic effect, erythropoietin requires a caution in its use so that hypertension due to more than necessary hematopoiesis should be prevented. In this sense, erythropoietin is a peptide drug belonging to category (b), which calls for frequent (for example, once in about a week) efficacy evaluation and dosage adjustment.
Meanwhile, since the activity of erythropoietin is dependent on its three-dimensional structure, it is essential, in the pharmaceutical manufacturing stage, to avoid formulations which might affect the spatial configuration of the peptide.
Therefore, should a long-acting preparation be developed that would insure a sustained efficacy of erythropoietin for a period of time corresponding to an interval of efficacy assessment (for example once in about a week), the current administration frequency of 3 times a week would be reduced to benefit the patient a great deal. This benefit not only should accrue to patients with renal anemia but also would be remarkable for patients in the field of surgery involving autologous blood transfusions where both the administration frequency and the dosage level could be decreased with great rewards. Moreover, it should be possible to minimize the abrupt increase in drug concentration immediately following administration and thereby suppress the excess reactions due to a precipitating onset of the pharmacologic action.
On the other hand, hyaluronic acid is a naturally-occurring acid mucopolysaccharide and has been used as an ethical drug such as an articular function-improving agent or an adjunct in ophthalmic surgery. It is known that an aqueous solution of hyaluronic acid has a high viscosity which generally retards diffusion of other substances.
In Japanese Patent Application Laid-open No. 62-129226 (1987) which corresponds to European Patent Publication No. 224,987, it is disclosed that a solution of hyaluronan including hyaluronic acid, its sodium salt, or hylan can release dissolved or dispersed pharmacologically active substances sustainedly due mainly to the viscosity of the solution of hyaluronan. In addition, those pharmacologically active substances that have cationic residues are diffused more slowly owing to an ionic exchange between the cationic residues and the carboxylic acid of hyaluronan. In the description of one of the working examples, tritium-labeled serotonin mixed with 0.1% aqueous solution of hyaluronic acid was put into an semi-permeable dialysis membrane bag (molecular weight cut off=10,000) and dialyzed against distilled water. The release rate of the tritium-labeled serotonin from the dialysis bag was reduced some ten-fold compared with the comparative example where no hyaluronic acid was employed.
By utilizing the above mentioned property, Japanese Patent Application Laid-open No. 1-287041 (1989) teaches a controlled release system containing hyaluronic acid or its pharmaceutically acceptable salt and, as particularly suitable physiologically active peptides, mentions insulin, crystalline insulin zinc, amorphous insulin zinc and glucagon. In the description of working examples, the above patent literature mentions as follows. When hyaluronic acid (molecular weight 1,400,000, viscosity method) was added to a neutralized injectable solution of swine insulin at a final concentration of 1% and the resulting composition was administered subcutaneously to normal male rabbits, an overt prolongation of hypoglycemia was found as compared with a positive control group of rabbits treated with insulin alone. Thus, this depressed blood glucose level was sustained at least till 12 hours following administration and, then, this effect had disappeared by 24 hours. The same literature contains a similar example for glucagon as well and mentions that the blood glucose resumed at latest by 8 hours after administration.
Japanese Patent Application Laid-open No. 2-213 (1990) also discloses a sustained release system for biologically active peptides which incorporates hyaluronic acid or its nontoxic salt. When, in working examples, a sustained release preparation of calcitonin or elcatonin containing 5% sodium hyaluronate was administered subcutaneouly to male rats, the depression of blood calcium persisted for a minimum of 12 hours. Similarly, when a sustained release system for human growth hormone containing 5% of sodium hyaluronate was administered to male rats, the blood human growth hormone level was sustained for at least 12 hours. In either case, prolongation of blood concentration was evident as compared with the corresponding comparative example in which sodium hyaluronate was not employed.
However, none of the above inventions allude to erythropoietin, whether by way of general description or as a working example.
These published inventions invariably utilize the phenomenon of delayed diffusion of ingredients in solutions of hyaluronic acid at the administration site, and Japanese Patent Application Laid-open No. 2-213 (1990) mentions that the most preferred concentration of hyaluronic acid itself is 3 to 7%. However, because of the high viscosity of the hyaluronic acid solution, removal of the air foam represents a serious technical challenge, calling for evacuation by centrifuging or decompression. Furthermore, also because of the high viscosity, it is necessary to employ a large-gauge needle for injection which gives the patient a pain which cannot be disregarded. Japanese Patent Application Laid-open No. 1-287041 (1989) is reticent about the concentration of hyaluronic acid used but contains working examples employing 1% hyaluronic acid. However, the precautions of the package insert for Artz (Kaken Pharmaceutical), which is a 1% sodium hyaluronate preparation for articular injection, recommend the use of a comparatively large-gauge needle of about 18 to 20 G. Therefore, the pain which these preparations for subcutaneous injection give to the patient is considerable.
Furthermore, Japanese Patent Application Laid-Open No. 3-4790 (1991) discloses an aqueous system that comprises polysaccharides, proteinases, and protein-like substances. It is disclosed that the system stabilizes the proteinases and prevent the loss of enzyme activity during storage especially at high temperature. In working examples, the combination of hyaluronic acid and either bovine serum albumin or gelatin prevented the loss of activity of esperase(Novo) produced by Becillus licheniformis better than the corresponding comparative examples in which either hyaluronic acid alone or protein alone was employed. However, this technique never alludes to sustained release of ethical drugs, especially erythropoietin, from the system when administered to living bodies.
Accordingly, there is a desire in the art to provide a long-acting medicine composition containing erythropoietin or other pharmacologically active substance which insures sufficient efficacy at a dosing interval of, for example, about one week through prolongation of drug release and which is lenient on the patient in terms of the pain associated with administration.